Jun-Yi Ge

Temperature dependence of lower critical field Hc1(T) shows nodeless superconductivity in FeSe

We investigate the temperature dependence of the lower critical field Hc1(T) of a high-quality FeSe single crystal under static magnetic fields H parallel to the c axis. The temperature dependence of the first vortex penetration field has been experimentally obtained by two independent methods and the corresponding Hc1(T) was deduced by taking into account demagnetization factors. A pronounced change in the Hc1(T) curvature is observed, which is attributed to anisotopic s-wave or multiband superconductivity. The London penetration depth λab(T) calculated from the lower critical field does not follow an exponential behavior at low temperatures, as it would be expected for a fully gapped clean s-wave superconductor. Using either a two-band model with s-wave-like gaps of magnitudes � 1 = 0.41 ± 0.1 meV and � 2 = 3.33 ± 0.25 meV or a single anisotropic s-wave order parameter, the temperature dependence of the lower critical field Hc1(T) can be well described. These observations clearly show that the superconducting energy gap in FeSe is nodeless.

Bound vortex dipoles generated at pinning centres by Meissner current

One of the phenomena that make superconductors unique materials is the Meissner-Ochsenfeld effect. This effect results in a state in which an applied magnetic field is expelled from the bulk of the material because of the circulation near its surface of resistance-free currents, also known as Meissner currents. Notwithstanding the intense research on the Meissner state, local fields due to the interaction of Meissner currents with pinning centres have not received much attention. Here we report that the Meissner currents, when flowing through an area containing a pinning centre, generate in its vicinity two opposite sense current half-loops producing a bound vortex-antivortex pair, which eventually may transform into a fully developed vortex-antivortex pair ultimately separated in space. The generation of such vortex dipoles by Meissner currents is not restricted to superconductors; similar topological excitations may be present in other systems with Meissner-like phases.

Vortex phase transition and isotropic flux dynamics in K0.8Fe2Se2 single crystal lightly doped with Mn

We have applied ac susceptibility measurements to study the vortex dynamics in a high quality K0.79Fe1.86Mn0.01Se2 single crystal with applied magnetic field both parallel and perpendicular to the c-axis. The irreversibility line has been determined in the frequency range of 133 Hz–9777 Hz which can be well fitted using Hirr=H0(1−T/Tc)β with β=1.49 for H∥c and β=1.63 for H∥ab. The field dependence of the activation energy U0 is derived in the framework of thermally activated flux creep theory, yielding a power law dependence ∼Hn with n≈−0.58 for H below 0.75 T and n≈−1 for H above 0.75 T. The derived activation energy U0 shows nearly isotropic behavior for both field directions and high values reaching 105 K at low fields. This observation points towards very strong intrinsic pinning in the investigated material, thus making KFeSe-systems promising for technical applications.

Direct observation of the depairing current density in single-crystalline Ba0.5K0.5Fe2As2 microbridge with nanoscale thickness

We investigated the critical current density (Jc) of Ba0.5K0.5Fe2As2 single-crystalline microbridges with thicknesses ranging from 276 to 18 nm. The Jc of the microbridge with thickness down to 91 nm is 10.8 MA/cm2 at 35 K, and reaches 944.4 MA/cm2 by extrapolating Jc(T) to T = 0 K using a two-gap s-wave Ginzburg-Landau model, well in accordance with the depairing current limit. The temperature, magnetic field, and angular-dependence of Jc(T,H,θ) indicated weaker field dependence and weakly anisotropic factor of 1.15 (1 T) and 1.26 (5 T), which also yielded the validity of the anisotropic Ginzburg-Landau scaling.

Magnetic dipoles at topological defects in the Meissner state of a nanostructured superconductor

In a magnetic field, superconductivity is manifested by total magnetic field expulsion (Meissner effect) or by the penetration of integer multiples of the flux quantum Phi(0). Here we present experimental results revealing magnetic dipoles formed by Meissner current flowing around artificially introduced topological defects (lattice of antidots). By using scanning Hall probe microscopy, we have detected ordered magnetic dipole lattice generated at spatially periodic antidots in a Pb superconducting film. While the conventional homogeneous Meissner state breaks down, the total magnetic flux of the magnetic dipoles remains quantized and is equal to zero. The observed magnetic dipoles strongly depend on the intensity and direction of the locally flowing Meissner current, making the magnetic dipoles an effective way to monitor the local supercurrent. We have also investigated the first step of the vortex depinning process, where, due to the generation of magnetic dipoles, the pinned Abrikosov vortices are deformed and shifted from their original pinning sites.

Nanoscale assembly of superconducting vortices with scanning tunnelling microscope tip.

Vortices play a crucial role in determining the properties of superconductors as well as their applications. Therefore, characterization and manipulation of vortices, especially at the single-vortex level, is of great importance. Among many techniques to study single vortices, scanning tunnelling microscopy (STM) stands out as a powerful tool, due to its ability to detect the local electronic states and high spatial resolution. However, local control of superconductivity as well as the manipulation of individual vortices with the STM tip is still lacking. Here we report a new function of the STM, namely to control the local pinning in a superconductor through the heating effect. Such effect allows us to quench the superconducting state at nanoscale, and leads to the growth of vortex clusters whose size can be controlled by the bias voltage. We also demonstrate the use of an STM tip to assemble single-quantum vortices into desired nanoscale configurations.

Giant increase of critical current density and vortex pinning in Mn doped KxFe2−ySe2 single crystals

We report a comparative study of the critical current density ( Jc) and vortex pinning among pure and Mn doped KxFe2−ySe2 single crystals. It is found that the Jc values can be greatly improved by Mn doping and post-quenching treatment when comparing to pristine pure sample. In contrast to pure samples, an anomalous second magnetization peak (SMP) effect is observed in both 1% and 2% Mn doped samples at T = 3 K for H∥ab but not for H∥c. Referring to Dew-Hughes and Kramers model, we performed scaling analyses of the vortex pinning force density vs magnetic field in 1% Mn doped and quenched pristine crystals. The results show that the normal point defects are the dominant pinning sources, which probably originate from the variations of intercalated K atoms. We propose that the large nonsuperconducting K-Mn-Se inclusions may contribute to the partial normal surface pinning and give rise to the anomalous SMP effect for H∥ab in Mn doped crystals. These results may facilitate further understanding of the super...

Dependence of the flux-creep activation energy on current density and magnetic field for a Ca10(Pt3As8)[(Fe1−xPtx)2As2]5 single crystal

We have performed detailed ac susceptibility measurements to investigate the vortex dynamics in a Ca10(Pt3As8)[(Fe1−xPtx)2As2]5 single crystal as a function of temperature, frequency, ac amplitude, and dc field. The field dependence of the activation energy U is derived in the framework of thermally activated flux creep theory, yielding a power law dependence of U ∼ Hα with α≈ −1.0 for H above 0.30 T, while below 0.3 T U is independent of the field. The activation energy reaches 104 K at low fields, suggesting strong pinning in the material. The nonlinear function of the activation energy vs. the current density is determined, which shows logarithmic dependence U(J)∝lnJ.

Flux pattern transitions in the intermediate state of a type-I superconductor driven by an ac field

The intermediate state of a type-I superconductor Pb film is studied by a scanning Hall probe and scanning ac-susceptibility microscopies under static and oscillating applied magnetic fields. The structure of the typical flux patterns during magnetic field penetration/expulsion shows a strong hysteresis. Under the action of an ac field, the multiply quantized flux tubes in a type-I superconductor reveal a dynamical reordering similar to what is observed in the Campbell regime for vortices in a type-II superconductor. Most strikingly, after shaking, higher density flux tube patterns demonstrate a reorganization from a superheated metastable tubular pattern to a stable stripe pattern. We provide direct experimental evidence that the flux reorganization behavior is a dynamical transition. The local distribution of the potentials providing pinning to intermediate state patterns is mapped out, which is, as far as we know, the first direct visualization of confinement of intermediate state domains by pinning centers.

Direct visualization of vortex ice in a nanostructured superconductor

Artificial ice systems have unique physical properties that are promising for potential applications. One of the most challenging issues in this field is to find novel ice systems that allow precise control over the geometries and many-body interactions. Superconducting vortex matter has been proposed as a very suitable candidate to study artificial ice, mainly due to the availability of tunable vortex-vortex interactions and the possibility to fabricate a variety of nanoscale pinning potential geometries. So far, a detailed imaging of the local configurations in a vortex-based artificial ice system is still lacking. Here we present a direct visualization of the vortex-ice state in a nanostructured superconductor. By using scanning Hall probe microscopy, a large area with the vortex-ice ground-state configuration has been detected, which confirms the recent theoretical predictions for this ice system. Besides the defects analogous to artificial spin-ice systems, other types of defects have been visualized and identified. We also demonstrate the possibility to realize different types of defects by varying the magnetic field.